The Java Language Specification, Third Edition

8.8.10 Preventing Instantiation of a Class CLASSES
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package p2;
class SonOfOuter extends p1.Outer {
void foo() {
new Inner(); // compile-time access error
}
}
The constructor for Inner is protected. However, the constructor is protected relative
to Inner, while Inner is protected relative to Outer. So, Inner is accessible
in SonOfOuter, since it is a subclass of Outer. Inner’s constructor is not accessible
in SonOfOuter, because the class SonOfOuter is not a subclass of Inner!
Hence, even though Inner is accessible, its default constructor is not.
8.8.10 Preventing Instantiation of a Class
A class can be designed to prevent code outside the class declaration from creating
instances of the class by declaring at least one constructor, to prevent the creation
of an implicit constructor, and declaring all constructors to be private. A
public class can likewise prevent the creation of instances outside its package by
declaring at least one constructor, to prevent creation of a default constructor with
public access, and declaring no constructor that is public.
Thus, in the example:
class ClassOnly {
private ClassOnly() { }
static String just = "only the lonely";
}
the class ClassOnly cannot be instantiated, while in the example:
package just;
public class PackageOnly {
PackageOnly() { }
String[] justDesserts = { "cheesecake", "ice cream" };
}
the class PackageOnly can be instantiated only within the package just, in
which it is declared.
DRAFT